How to make x-axis mirror work on newly created bones? (2.79)
In order for the Mirror Option to know what it should mirror, you need to name the bones according to some guidelines on the wiki (https://docs.blender.org/manual/en/latest/animation/armatures/bones/editing/naming.html). There are also Tools to for example flip the side in the name or mirror bones and their names. You find them in the Armature Menu > Names.